I don't really understand the delay.

Is the reason to make bigger difference with Mimas so the owners will be more likely to upgrade to Alita? I highly doubt anyone who owns Mimas will be wiling to shelf that kind of money for upgrade to Alita with or without updates. If anything they will be angry if only Alita gets the updated software and not Mimas.
As incentive for new buyers? The Goodereader review was positive even on current SW and they can always do the updates with already released device?
The HW gets out of date pretty fast and each month that already finished device don't get on the shelf the competition could come out in with devices with even more compelling specs.